1. Field of the Invention
The present invention relates to a configuration for separating a pair of sheet conveying rollers which convey a sheet and which can separate from each other, and more particularly, to a sheet conveying apparatus, a sheet processing apparatus, and an image forming apparatus capable of reducing an impact force caused when a pair of sheet conveyance rotating members move from separated positions to an abutted position.
2. Description of the Related Art
A conventional image forming apparatus of an electrophotographic system such as a copying machine, a laser beam printer, a facsimile machine and a multifunctional machine thereof includes an image forming portion which forms a toner image on a sheet, and a fixing portion which fixes the toner image formed on the sheet. Some image forming apparatuses include a sheet processing apparatus which carries out bookbinding processing for a sheet on which a toner image is fixed. A conventional image forming apparatus and a sheet processing apparatus include a sheet conveying apparatus which conveys a sheet.
It is generally known that when bookbinding processing is carried out by the conventional sheet processing apparatus, after a predetermined number of sheets are superposed on each other to form a sheet bundle and then, central portions of the sheet bundle are bound by strings, staples, adhesives or the like, and binding portions are folded in the middle, thereby forming the sheet bundle into a booklet form. Some of the conventional sheet processing apparatuses include a folding plate and two pairs of folding rollers arranged side by side in a sheet-pushing direction for carrying out the bookbinding processing. When sheets are folded in the middle, binding portions of the sheet bundle are sequentially pushed into nip portions of a pair of upstream folding rollers and a pair of downstream folding rollers in the sheet-pushing direction by the folding plate, thereby folding the sheet bundle in the middle.
In the case of the sheet processing apparatus of such a configuration, if the number of sheets is increased, when the folding plate is moved backward after the folding operation which is carried out by pushing the sheet bundle by the folding plate is completed, a moving load at the time of the backward moving operation is increased by a nip pressure of the pair of folding rollers. Hence, to reduce the moving load, the pair of folding rollers located upstream in the sheet-pushing direction are separated from each other after the folding operation, thereby opening a space between the pair of rollers.
To separate the pair of folding rollers from each other, a pair of swinging members support two pairs of folding rollers, and a turning center of the swinging members is set downstream from the pair of downstream folding rollers in the sheet-pushing direction. By the roller moving mechanism having such a configuration, when a sheet bundle reaches the pair of downstream folding rollers, the pair of upstream folding rollers are moved in the separating direction by an arm ratio between a sheet bundle thickness and the turning center of the swinging members (see Japanese Patent Laid-Open No. 2004-224554).
In the conventional sheet conveying apparatus, the pair of folding rollers whose abutting pressure of the rollers is set relatively high and a pair of thermal fixing rollers are moved between the abutted position and the separated positions by the roller moving mechanism having the swinging member and a cam mechanism.
In the conventional sheet conveying apparatus having the roller moving mechanism, in the case of the roller moving mechanism disclosed in Japanese Patent Laid-Open No. 2004-224554, the single swinging member supports the plurality of (pair of) folding rollers. In the case of such a configuration, the pair of folding rollers are moved in the separating direction only by the thickness of the sheet bundle. Therefore, the moment a rear end of the sheet bundle passes through the pair of downstream folding rollers, the pair of upstream and downstream folding rollers simultaneously try to move in the abutting direction by the spring effect.
Also some of conventional roller moving mechanisms include an eccentric cam on one of roller ends as a unit configured to separate and abut a pair of heat fixing rollers from and against each other (see Japanese Patent Laid-Open No.10-247028). Due to characteristics of a cam mechanism, a load direction of a rising side of a cam curve (moving direction in which a cam follower separates from the center shaft) and a load direction of a lowering side of the cam curve (moving direction in which the cam follower approaches the center shaft) are changed at the top dead center (point of the cam follower which is furthest from the center shaft) as a boundary.
That is, a rising side of the cam curve becomes drag when the cam follower is pushed up by a pressurizing force for contacting a pair of rollers under pressure, and a lowering side of the cam curve becomes an assisting force for pushing down the cam follower by a pressurizing force of the pair of rollers. Therefore, the tooth abutting surface moves between backlashes of a gear in a driving transmission portion at a load changing point at this top dead center as a boundary. Especially when a gear is provided on the cam shaft or the like, the load changing point of the cam and a phase angle of a teeth where a teeth abutting surface of a gear match with each other. As a result, a repeatedly impact load is applied to a specific teeth, and it is considered that there are problems that a teeth surface is damaged and a collision sound is generated.